West Bromwich Albion have said that Jake Livermore confronted a West Ham United supporter in the crowd because of a comment made about the death of the midfielder's son.
West Ham say they have identified the fan, and an investigation is underway to establish whether he directed abuse about the England international's baby son, who died in 2014.
Livermore, who was substituted just after the hour, had to be escorted down the tunnel following the incident, which was not clearly picked up on CCTV footage. West Ham acted quickly to eject the fan, who was spoken to by stadium staff on Tuesday evening.
West Brom released a statement on Wednesday which reads: "In the light of intense media speculation, West Bromwich Albion would like to clarify an incident involving our player Jake Livermore in last night's Premier League game at West Ham.
"Following his substitution in the second half, Jake was subjected to general abuse from home supporters which he shrugged off. However, he acknowledges that he moved to confront one spectator who chose to make a comment about the death of his infant son.
"The Club would like to think that all right-minded football supporters could understand this reaction which did not result in any physical exchanges before Jake was led away to our dressing room.
"Jake is an outstanding young individual who has the total support of everyone at the Club and he has made it clear he considers this statement to be an end to an unhappy incident.
"Albion were very appreciative of the swift response by the officials of West Ham in dealing with this matter and are more than happy to leave any further action they deem appropriate in their hands.
"The Club has responded to an FA request to offer our observations of the incident and will not be making any further comment."
Press Association Sport reports that the London club are continuing to actively look into the matter, working with West Brom and 28-year-old Livermore to determine exactly what happened.
The Football Association has also requested observations from all parties on the incident. Once the investigations are concluded, West Ham intend to take swift action regarding the individual concerned, with a stadium ban being considered.
Baggies boss Alan Pardew said after his team's 2-1 defeat that he had seen Livermore among supporters in the crowd, but did not know what had provoked the midfielder's actions.
"The only thing I know is I see Jake in the crowd, which disturbed me. Obviously you don't want to see a player in the crowd," Pardew said.
"There's no way he's going in the crowd -- because I know him, he's a great lad -- unless he was provoked severely. That's all I know.
"I haven't had a chance to talk to Jake. I'll speak to Jake and we'll go from there."
Livermore joined West Brom from Hull on a four-and-a-half-year deal in January 2017, which is said to have cost the Baggies around £10 million.
The former Tottenham Hotspur midfielder tested positive for cocaine after Hull's Premier League win over Crystal Palace during April 2015 and played three further matches before being banned by the FA.
Livermore, though, subsequently escaped a further suspension after the FA ruled the death of his new-born son Jake Junior in May 2014 was an overwhelming mitigating factor.
An FA statement at the time read: "Following an Independent Regulatory Commission hearing, Hull City FC midfielder, Jake Livermore, has admitted breaching Regulation 3 of The FA's Anti-Doping Regulations after testing positive for a prohibited substance.
"However, due to the specific and unique nature of the circumstances surrounding the case, he will face no suspension."
Livermore was ordered to undertake a course of rehabilitation and education, and was also the subject of target testing for a period of 12 months.